Perspectives on OLED Technology

We are approaching forty years since Tang and Van Slyke reported the first example of electroluminescence from an organic light-emitting diode (OLED) operating at relatively low voltages.¹ This pioneering research from their labs at Kodak used a green fluorescent aluminum complex, Alq₃ as the emitter. Significant strides have since been made in both OLED materials and device development, where today, OLEDs can be found in myriad consumer electronic devices and beyond, from smartwatches, mobile phones, tablets, televisions, commercial displays, and automotive lighting. Despite the maturity of this technology, innovation in this space continues, driven by an ever-expanding search for sustainable, high-performance organic semiconductor materials. Thus, the 27 research articles comprising this themed issue of J. Mater. Chem. C. cover examples of emitters for both solution-processable and vacuum-deposited OLEDs, including different flavours of thermally activated delayed fluorescent (TADF) materials, phosphorescent organometallic complexes, and inverted singlet–triplet gap compounds, commonly known as INVEST materials.

Two reviews and a perspective are included in this themed collection. Iyer and co-workers provide an overview of colour tuning strategies in TADF emitters (https://doi.org/10.1039/D5TC02697K). Ren and Su summarize recent progress in small-molecule solution-processable TADF emitters (https://doi.org/10.1039/D5TC02897C). Krzywiecki and co-workers provide a perspective on the challenges of interface engineering for efficient charge injection from ZnO and metal phthalocyanines in OLEDs (https://doi.org/10.1039/D5TC02786A).

Several theoretical studies graced this themed issue and highlight the importance of modelling in emitter design. De Silva and co-workers reported a computational study that predicted that azaphenalene-based molecules would show inverted singlet–triplet gaps. Their simulations revealed that very small negative ΔE_ST do not always reduce delayed fluorescence lifetimes. Instead, tuning the vibronic coupling and molecular symmetry are crucial to balance radiative and non-radiative transitions for optimal performance (https://doi.org/10.1039/D5TC01155H). A separate study on INVEST compounds from Sancho-García and co-workers described that double hybrid DFT methods can accurately predict ΔE_ST in the NAH159 dataset of low-gap organic molecules (https://doi.org/10.1039/D5TC01799H). Song and co-workers demonstrated a machine learning framework for predicting and designing circularly polarized TADF (CP-TADF) molecules having high |g_lum| values. By combining statistical analysis, molecular fingerprints, and a generative variational autoencoder, the model effectively bridges theoretical and experimental discrepancies. This approach provides a tool for CP-TADF materials predicted to show high dissymmetry values and synthetic accessibility (https://doi.org/10.1039/D5TC02451J).

The challenge of designing bright and stable blue emitters for OLEDs remains a popular subject. Yan, Zhou, Wei, Chi and co-workers report homoleptic carbene-based iridium(III) complexes showing narrowband emission (https://doi.org/10.1039/D5TC01314C). The devices showed outstanding external quantum efficiencies (EQE), which could be improved using a hyperphosphorescent device architecture using ν-DABNA as the terminal emitter. Rajamalli and Yadav reported a xanthene-anthracene-based small molecule (PhAn-Xn) that can function as both an emitter and a host for ultra-deep-blue OLED applications. The blue OLED fabricated using neat PhAn-Xn showed an EQE_max of 4.2% (https://doi.org/10.1039/d5tc02452h). Zhao and co-workers investigated the impact of acceptor on the device stability using two blue TADF materials incorporating the same stable pyridine-derived donor but different acceptors of xanthone and triphenyltriazine (https://doi.org/10.1039/D5TC01399B). Of the two, the use of the xanthone-based emitter translated into more stable OLEDs. Duan and co-workers explored the incorporation of B–N covalent bonds within multi-resonant TADF (MR-TADF) emitter design, reporting sky-blue OLEDs showing outstanding peak brightness and minimal efficiency roll-off (https://doi.org/10.1039/D5TC01905B). Zysman-Colman and co-workers explored the use of saturated analogs of carbazole as moieties within sky-blue MR-TADF emitter design (https://doi.org/10.1039/D5TC02496J). Despite the photoluminescence quantum yields being very high in toluene, these drop significantly in 1 wt% doped films in mCP, and so OLEDs were not fabricated. Sasabe and co-workers demonstrated the use of peripheral mesityl groups as sterically blocking moieties that also conferred preferential horizontal orientation of the transition dipole moment of a sky-blue MR-TADF emitter (https://doi.org/10.1039/D5TC02609A). The corresponding OLED thus showed outstanding maximum EQE, EQE_max, of 38.6%. Wang and co-workers reported the use of a decorated indolocarbazole that shows deep blue, narrowband emission (https://doi.org/10.1039/D5TC02735G). Owing to the use of a high-lying reverse intersystem crossing (hRISC) exciton harvesting mechanism, the OLEDs showed an EQE_max that surpasses the 5% limit for fluorescent devices. Nutalapati, Lee, Konidena and co-workers similarly exploit an hRISC mechanism in their devices, using two deep-blue organic emitters based on phenanthroimidazole frameworks (https://doi.org/10.1039/D5TC02737C). Xie, Ma, Tang, Li and co-workers modulate the performance of a triazatruxene-DOBNA type emitter through the incorporation of peripheral pyridyl groups within the triazatruxene donor (https://doi.org/10.1039/D5TC02857D). Such a design leads to emitters showing fast RISC and blue devices showing milder efficiency roll-off. Azari, Soldano and co-workers explored devices of varying architectures using 2CzPN, a blue donor–acceptor TADF emitter, doped in a high-triplet-energy DPEPO host (https://doi.org/10.1039/D5TC02796A). The optimal emission was found for the ∼10 wt% doped films of 2CzPN in the DPEPO host. Two-layer organic light-emitting transistor devices exhibited intrinsic blue emission, whereas the addition of an electron-transport layer resulted in a red-shift and the emergence of an additional exciplex emission at the emissive layer/electron-transport interface. Yang and Mei et al. developed a series of blue and sky-blue TADF emitters using tert-butylcarbazole or bicarbazole donors and a pyridylacridone acceptor (https://doi.org/10.1039/D5TC03112E). Their design strategy was one that simultaneously optimized fast RISC and high Φ_PL. The sky-blue devices achieved an outstanding EQE_max of 34.4%.

Several studies focused on solution-processable emitter design for solution-processed OLEDs. Yu, Wang, Zhang and co-workers synthesized a solution-processable quasiplanar TADF blue emitter (λ_PL of 440 nm) whose design is based on a dual-locking strategy via intramolecular O–H⋯O hydrogen bonds (https://doi.org/10.1039/D5TC02547H). The solution-processed OLED achieved an EQE_max of 10.3%. Promarak and co-workers developed two solution-processable triplet-exciton-harvesting emitters (CPhCN and CBzTPA) by linking a soluble tercarbazole donor dendron to a donor–acceptor fragment (https://doi.org/10.1039/D5TC02980E). CPhCN is a TTA-based sky-blue emitter, while CBzTPA is an HLCT-based orange emitter. The OLED with an emissive layer composed of a CPhCN : CBzTPA (99.24 : 0.76) blend achieved stable white emission (CIE 0.315, 0.372) with an EQE_max of 9.43%. Hua, Yan, Ren and co-workers used a backbone-engineering strategy by incorporating carbazole and dibenzofuran units with varying loadings of a naphthalimide-dimethylacridine unit (https://doi.org/10.1039/D5TC03166D). The linkage position of dibenzofuran (3,7- vs. 2,8-) modulated both energy transfer and exciton dynamics. The optimized polymer (pNAI-DBF3705) has a Φ_PL of 78%, a fast k_RISC of 8.76 × 10⁵ s⁻¹, and the solution-processed OLEDs showed an EQE_max of 10%. The physical interactions between the host and emitter in solution-processed OLEDs significantly influence device performance. Cho, Choi and co-workers reported the blue-emissive solution-processable MR-TADF emitter, 4FlDABNA, which has a sterically protected emissive core to mitigate aggregation-caused quenching (https://doi.org/10.1039/D5TC01813G). In particular, the combination of this emitter with a novel host, Cy-2(Ph-mCzCz) in the emissive layer provided an optimal balance between molecular stability, thin-film morphology, and device efficiency, demonstrating the latter as a promising host for solution-processed MR-TADF OLEDs.

Modulating charge-transfer properties is frequently a key driver for optimizing the photophysics of emitter systems. Liu, Wang, and co-workers showed that tuning through-space charge transfer (TSCT) is key to optimizing the performance of TADF emitters (https://doi.org/10.1039/D5TC00953G). Among the devices presented, that with 2PhTPA achieved the highest EQE_max (17.9%) due to balanced TSCT and improved exciton dynamics, highlighting TSCT control as a viable strategy to enhance device efficiency. Brütting and Unni and co-workers studied an exciplex system consisting of NPB (a commonly used HTM) and OXD-7 (a commonly used ETM) (https://doi.org/10.1039/D5TC01944C). The dual functionality of the NPB:OXD-7 exciplex for OLEDs and photodetectors was explored, with consideration of the influence of surface potential in organic semiconductor films. This work pioneered the exploration of multifunctional capabilities in exciplex-type OLEDs, expanding their role beyond light emission.

Examples of chiral emitters were also featured in this themed issue. Chao, Stará, Chou, Wong and co-workers designed a chiral helicene–perylene diimide emitter with excellent molecular rigidity that exhibits red emission (630 nm) and CPL behavior (|g_lum| ≈ 10⁻³) in solution (https://doi.org/10.1039/D5TC02788H). The incorporation of this emitter as a dopant dispersed in the exciplex-forming co-host matrix afforded red-emitting CP-OLEDs with an EQE_max of 1.41% and |g_EL| of 1.1 × 10⁻³. These results demonstrated that the synergistic combination of TADF-enabling exciplex-based co-hosts and chiral helicene emitters to achieve CP-OLEDs, is a versatile approach for chiroptical devices. Varganov, Würthner and Chalifoux and co-workers reported fluorescent syn- and anti-dinaphthopentacenes (https://doi.org/10.1039/D5TC02535D). The chiral syn isomer showed superior photostability, whereas the planar anti isomer delivered devices showing much higher fluorescence efficiency (>2000 cd m⁻²) as compared to the syn-counterpart (250 cd m⁻²), underscoring the impact of structural isomerism on OLED performance. Liao, Jiang, Chen, Li and co-workers reported a spiro-fused chiral material for high efficiency deep-blue OLEDs. The resulting device showed an EQE_max of 15% and electroluminescence with a dissymmetry factor of 2.1 × 10⁻³ (https://doi.org/10.1039/d5tc03265b).

The themed issue also highlighted a number of distinct emitter designs. Zhang and Zheng and co-workers reported color-tunable emitters by incorporating naphthalene, pyrene, anthracene, and perylene units into an MR-TADF emissive core (https://doi.org/10.1039/D5TC02297E). These compounds show narrowband fluorescence from green to red (509–605 nm) with small FWHM values (26–32 nm) and high Φ_PL (as high as 95%). With the help of a TADF sensitizer, the fabricated OLEDs using these emitters achieved EQE_max values of 19.5-23%. Wang, Lu and co-workers developed MR-TADF emitters that incorporated spiral electron-acceptor fragments that balance short-range and long-range charge-transfer characteristics (https://doi.org/10.1039/D5TC02393A). The OLED with a green emitter (λ_PL at 512 nm, FWHM of 36 nm), achieved an EQE_max of 36.1% and abated efficiency roll-off. Efficient non-doped bluish green-to-green TADF-OLEDs were reported by Li, Chan, and co-workers using two TADF emitters, HDT1 and 36Me-HDT1 (https://doi.org/10.1039/D5TC02141C). The devices achieved EQE_max values of 16.5 and 21.0%, respectively. Further, single-stack, three-color, warm-white hyperfluorescent OLEDs were demonstrated with an EQE_max of 12.1%.

References

1. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913–915.

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